Polarization splitter and rotator

ABSTRACT

Example polarization splitter and rotator devices are described. In one example, an optical apparatus includes a splitter configured to split a light signal into a first signal having a first polarization and a second signal having a second polarization, a polarization rotator configured to rotate the second polarization of the second signal into a third polarization, and a polarization mode converter configured to convert the third polarization of the second signal into the first polarization. In certain aspects of the embodiments, the splitter can be a curved multi-mode inference (MMI) polarization splitter, and the polarization rotator comprises input and output ports, with the output port being wider than the input port. The polarization mode converter can be an asymmetrical waveguide taper mode converter. The devices described herein can overcome the deficiencies of conventional devices and provide low insertion loss, flat and/or wide wavelength response, high fabrication tolerance, and compact size.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to optical waveguide devices,and more particularly, to optical waveguide devices that employpolarization splitters.

BACKGROUND OF THE DISCLOSURE

A polarization splitter and rotator (PSR) can be a passive device usedin integrated optics, such as a data transmitter or receiver (e.g., atransceiver). For example, a PSR may use dual polarization divisionmultiplexing (DPDM) to double the bandwidth of a transceiver. A PSR mayalso be used to build polarization insensitive receivers, which candetect signals transmitted in optical single mode fibers.

When a light signal is input into a waveguide of a photonic integratedcircuit (PIC), polarization of the light signal may be known based uponthe input circuitry. However, when a light signal is received by areceiver, polarization of the light signal may be unknown. Indeed, thepolarization may be a random polarization or an unknowntransverse-electric (TE), transverse-magnetic (TM), or TE/TMpolarization. Therefore, a PSR may use directional couplers to split alight signal into TE and TM polarizations and to rotate the light into aknown TE state so that data can be retrieved from the light signal.

However, conventional PSRs may suffer from a narrow wavelength response,high insertion loss, sensitivity to fabrication error, and a large size.The coupling ratio of directional couplers may be wavelength sensitiveand it may be difficult to obtain a flat wavelength response. Moreover,associated polarization rotators and mode converters may be relativelylarge in size and exhibit optical attenuation.

FIG. 1 shows a conventional PSR 100. PSR 100 may include adirectional-coupler-based polarization splitter 110, a bi-leveltaper-based TM₀-to-TE₁ polarization rotator 120, and an asymmetricMach-Zehnder-Interferometer (MZI) based TE₁-to-TE₀ mode converter 130.Light signals having both TE₀ (i.e., zero order TE mode) and TM₀ (i.e.,zero order TM mode) polarizations may be input via input port 101. Forthe TE₀ input, TE₀ polarized light may travel to thru-port 111 directly.For the TM₀ input, TM₀ polarized light may be coupled to cross-port 112and then gradually converted to a TE₁ (i.e., first order TE mode)polarization mode when traveling through polarization rotator 120, whichmay provide a bi-level taper. The output TE₁ polarization mode may thenbe split into two TE₀ mode beams by converter 130, which may alsointroduce an extra phase difference of π between the two beams and thenphase align the beams so they can be converted into a TE₀ polarizationmode output from converter 130.

FIG. 2 shows another example of a conventional PSR 200. The operationsequence in this example is polarization rotation, splitting, and modeconversion. TE₀/TM₀ mixed polarization light may be input at input port201. As the input light propagates through a taper waveguidepolarization rotator 210, TM₀ mode light may be converted to TE₁ modelight, while the TE₀ mode light maintains unchanged. The TE₁ and TE₀modes may then be input into a mode splitter and rotator 220 at input211, and then separated into two beams by the mode splitter and rotator220. Mode splitter and rotator 220 may be a Y-junction, directionalcoupler, or MZI interferometer, for example. The TE₀ mode light may beoutput at through port 221. The TE₁ mode light may be converted to theTE₀ mode using mode transition or mode interference in the mode splitterand rotator 220 and output at cross port 222.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to facilitate a fuller understanding of the present disclosure,reference is now made to the accompanying drawings, in which likeelements are referenced with like numerals. These drawings should not beconstrued as limiting the present disclosure, but are intended to beillustrative only.

FIG. 1 shows an exemplary conventional PSR according to one example ofthe present disclosure.

FIG. 2 shows another exemplary conventional PSR according to one exampleof the present disclosure.

FIG. 3 shows an integrated optical apparatus in accordance with someembodiments of the present disclosure.

FIGS. 4A-4E show diagrams associated with a polarization splitter inaccordance with some embodiments of the present disclosure.

FIGS. 5A-5D show waveguide thickness variation versus transmission inaccordance with some embodiments of the present disclosure.

FIGS. 6A-6D show diagrams associated with a polarization converter inaccordance with some embodiments of the present disclosure.

FIGS. 7A-7D show simulation diagrams of polarization tolerance inaccordance with some embodiments of the present disclosure.

FIGS. 8A-8C show converter performance diagrams in accordance with someembodiments of the present disclosure.

FIGS. 9A-9B show simulations of fabrication tolerance of a converter inaccordance with some embodiments of the present disclosure.

FIGS. 10A-10E show PSR performance in accordance with some embodimentsof the present disclosure.

FIG. 11 shows how the presented PSR design can be applied in apolarization insensitive receiver.

FIG. 12 shows how the presented PSR design can be applied in apolarization-mux transmitter.

DETAILED DESCRIPTION OF EMBODIMENTS

The present disclosure and the related advantages are described andhighlighted in the following description and accompanying figures whichare not necessarily drawn to scale. Detailed descriptions of structureand processing techniques are omitted so as to not unnecessarily obscurethe present disclosure. Further, in the following description, numerousspecific details are set forth regarding the systems and methods of thedisclosed subject matter and the environment in which such systems andmethods may operate in order to provide a thorough understanding of thedisclosed subject matter. It will be apparent to one skilled in the art,however, that the disclosed subject matter may be practiced without suchspecific details. Further, certain features, which are well known in theart, are not described in detail in order to avoid complication of thedisclosed subject matter. In addition, it will be understood that theexamples provided below are exemplary and other systems and methods arewithin the scope of the disclosed subject matter.

As discussed above, the conventional PSRs 100 and 200 of FIGS. 1 and 2may suffer from drawbacks. For example, both may exhibit a small (e.g.,1 decibel (dB)) bandwidth, where the wavelength range of the bandwidthhas an optical loss variation of less than 1 dB, for example, and may be30 to 40 nanometers (nm), for example. Also, both the PSRs 100 and 200may have relatively high insertion loss (e.g., 1.5 to 2.5 dB) andsensitivity to fabrication errors. In addition, in order to workproperly, the PSRs 100 and 200 may have to mix the TE₀/TM₀ mode lightbeams, instead of processing them independently, which can make dualpolarization division multiplexing (DPDM) difficult.

Therefore, there may be a need for PSRs that overcome the deficienciesof conventional devices, and that may include, for example, lowinsertion loss (e.g., less than 1 dB), flat and/or wide wavelengthresponse, high fabrication tolerance, and compact size. Such advancesmay be applicable to photonic transceivers, for example, among otherrelated devices.

Compared to conventional systems, the embodiments of the presentdisclosure achieve various improvements. First, embodiments of thepresent disclosure may use an MMI-based polarization splitter ratherthan directional coupler-based splitters, which may achieve a flatterwavelength response and improved fabrication tolerance. Second, the MMIsplitter may not be a straight MIMI or a quadratic-curve MMI, but may bea particle-swam-optimized MMI, which may achieve low loss, have acompact size, have a large 1 dB bandwidth, and have acceptablefabrication tolerance—all simultaneously. Third, embodiments of thepresent disclosure may employ an asymmetrical waveguide taper to replacean interferometer-based mode converter, which may improve fabricationtolerance and reduce optical loss. Embodiments of the present disclosuremay provide improved performance that is better than conventional PSRs.Embodiments of the present disclosure may be relevant to opticalreceiver and transceivers, such as coherent transceivers, among otheroptical communications devices.

FIG. 3 shows an integrated optical apparatus 300 in accordance with someembodiments of the present disclosure. Optical apparatus 300 is a PSRand includes a particle swarm optimized curved multi-mode-interference(MMI) polarization splitter 310, a bi-level waveguide polarizationrotator 320, and an asymmetric waveguide taper mode order converter 330.The incoming light signal at input 301 may be an unknown mix of TE₀ andTM₀ polarized light. The MMI polarization splitter 310 splits the TE₀mode light from the TM₀ mode light and outputs them separately atthrough port 311 and cross port 312. The bi-level-waveguide-basedpolarization rotator 320 rotates the TM₀ mode light into TE₁ mode light,which is input into asymmetrical waveguide taper mode converter 330 atinput 321. Asymmetrical waveguide taper mode converter 330 furthertransfers the TE₁ mode light into TE₀ mode light, which is output atport 331. Thus, two TE₀ mode light beams may be output (one at port 311and the other at port 331), and each of the beams is independent of eachother. The output light may be received by one or more receivers, suchas a receiving photodetector. In some embodiments, the one or morereceivers may include a diversity receiver.

As described in further detail below, the MMI-based polarizationsplitter 310 may be less wavelength-sensitive and more fabricationtolerant compared to conventional splitters. The MMI-based polarizationsplitter 310 may exhibit a flat spectrum bandwidth in its outputsignals. In one example, the MMI-based polarization splitter 310 isfabricated on a silicon layer and covered with silicon dioxide cladding.The silicon layer can be 220 nm thick. Below the silicon layer is aburied oxide (BOX) layer. The BOX layer can be about 2 micron (μm)thick. To enable a 1 dB transmission bandwidth that is greater than 70nm, the width of the MMI-based polarization splitter 310 may be made assmall as possible. However, the left and right edge of the MMI-basedpolarization splitter 310 may be sized wide enough to accommodate two420 nm wide waveguides, as well as a waveguide gap larger than 300 nm.The MMI-based polarization splitter 310 can include a curved rib layerof the 220 nm thick silicon. Besides the rib layer, no other siliconlayer with a different thickness is used.

FIG. 4A shows an example structure 400 of the MMI-based polarizationsplitter 310 shown in FIG. 3. The structure 400 includes inputwaveguides 410, an MMI polarization splitter 420, and output waveguides430. In FIG. 4A, the incoming light signal at one of the inputwaveguides 410 may be an unknown mix of TE₀ and TM₀ polarized light. TheMMI polarization splitter 420 splits the TE₀ mode light from the TM₀mode light and outputs them separately at the output waveguides 430,which correspond to ports 311 and 312 in FIG. 3. In one example, the MMIpolarization splitter 420 can include a curved rib layer of 220 nm thicksilicon. The MMI polarization splitter 420 can be split into a number ofsections, such as 34 sections, as also shown in FIG. 4A and described inadditional detail below.

Other MMI designs including curves have been based on exponential orquadratic curves. Those designs tend to violate the adiabatic criterionand bring significant optical loss. Although employing adiabaticcriterion could help minimize the loss, it potentially leads to bulkydimensions. For an ideal polarization splitter, compact size, largeextinction ratio, high fabrication tolerance, and a broad 1 dB bandwidthare also highly desired. Unfortunately, these figures of merit have notbeen typically considered in curved MMI designs. Hence, the traditionalexponential or quadratic curve MMIs are not suitable. There aretradeoffs associated with polarization splitters, such as the competinginterests of low insertion loss and flat wavelength response. Anothercompeting interest is high fabrication tolerance and compact size. Thepolarization splitters of the present disclosure exhibit compact size, arelatively high fabrication tolerance, a large 1 dB bandwidth, a highextinction ratio, and a low insertion loss, all simultaneously.Numerical optimization can be relied upon to consider these trade-offsand reach a final optimal design according to the embodiments describedherein. The Particle Swarm Optimization (PSO) method, for example, canbe relied upon, setting all design parameters as variables.

In one example, the MMI polarization splitter 420 may use a curvy MMIcoupler that may be divided into a plurality of sections. For example,the curvy MMI coupler may be divided into 34 sections, as shown in FIG.4A. The width of each section, as well as the position of the inputand/or output waveguides may be adjusted and optimized using the PSOmethod. The PSO method can be relied upon to assess all the designvariables, review different designs, calculate the above-mentionedfigures of merit of the designs, compare the results, and choose thebest design, in one iteration. Then, the next iteration will be done byreferring to the results of the previous iteration to further improvethe design. After hundreds of iterations, the optimal MIMI polarizationsplitter design can be reached.

The design of the MMI polarization splitter 420 can also be optimized insimulation, such as through simulation using the 3D FDTD software toolof Lumerical, Inc. of Vancouver, British Columbia, Canada. FIGS. 4B and4D show the optical power propagation of TE₀ and TM₀ light in thesimulation area based on FDTD simulation results. In FIGS. 4B and 4D,the X axis stands for the coordinates in propagation direction and Yaxis stands for the coordinates vertical to the propagation direction inthe simulation area. By observing FIGS. 4B and 4D, it is possible to seehow the power of the TE₀ and TM₀ mode light has split. As shown in theFDTD diagram in FIGS. 4B and 4D, the TM₀ mode light was successfullysplit from the TE₀ mode light, into upper and lower branches of theoutput waveguides 430, respectively. The polarization-mixed incomingsignals are input from the upper input waveguide 410 into the MMIpolarization splitter 420. The TE₀ mode light is output in theright-bottom branch of the output waveguides 430, and the TM₀ mode lightis output in the right-top branch of the output waveguides 430.

FIGS. 4C and 4E show the calculated optical transmission rate of the TM₀and TE₀ mode light, respectively, corresponding to the diagrams of FIG.4B and FIG. 4D. The optical transmission was measured from 1270 to 1340nm with a step of 0.1 nm by using the power monitor in the FDTD softwaretool. By inputting the TE₀ and TM₀ light at the upper input waveguide410 and reading the transmission of TE₀ and TM₀ light at the lower andupper branches of the output waveguide 430, it is possible to calculatehow much the optical loss and extinction ratios is for the TE₀ and TM₀mode light quantitatively. In FIG. 4C, the X axis stands for thewavelength in nm, and the Y axis stands for the absolute transmissionrate (e.g., 0.9 means 90%). For example, the extinction ratio for bothTE₀ and TM₀ at 1310 nm is greater than 20 dB, and the insertion loss isonly 0.45 dB (i.e., 90.17%). The 1 dB bandwidth is far larger than 70nm. In some embodiments, the MMI polarization splitter 310 may be sizedat or about 17.12 μm long and at or about 1.5 μm wide. The FIG. 4diagrams may correspond to such a sized design.

As shown by FIGS. 5A and 5B, the fabrication tolerance of the MMIpolarization splitter 310 has been examined over a waveguide thicknessin the range of ±10 nm and over the waveguide width range of ±30 nm. Therange is chosen according to an exemplary fabrication error range. Forexample, FIGS. 5A and 5B show that when the waveguide thickness variesfrom −10 nm to +10 nm, TM₀ transmission varies less than 6% and TE₀transmission varies less than 5%, which shows that the transmission maybe tolerant. FIGS. 5C and 5D show that when the waveguide width variesfrom −30 nm to +30 nm, the TM₀ and TE₀ transmission may change comparedto FIGS. 5A and 5B. For example, when the width variation is within ±10nm, the TM₀ and TE₀ transmission changes less than 10% and 15%respectively. In some embodiments, this tolerance is acceptable.

FIG. 6A shows an example structure 600 that can be used for the bi-levelwaveguide polarization rotator 320 shown in FIG. 3, for example, inaccordance with some embodiments of the present disclosure. Thestructure 600 may have asymmetry where input port 610 is less widecompared to output port 630. Area 620 may reflect a rib layer ofstructure 600, for example.

FIG. 6B shows another diagram of the exemplary structure 600 inaccordance with some embodiments of the present disclosure. For example,the port 610 may be at or about 420 nm wide. The output port 630 may beat or about 1000 nm wide, for example. The structure 600 may be adouble-etched structure, including a ridge layer 660, which may besilicon, and a rib layer 620, which may also be silicon. In one examplecase, the ridge layer 660 may be 220 nm thick. For example, rib layer620 may be at or about 90 nm thick. For example, rib layer 620 may be ator about 420 nm wide in a first region and may taper outward, forexample, as the layer proceeds toward output port 630. For example,ridge layer 660 may be at or about 420 nm wide in a first region and maytaper outward to 530 nm and then 1000 nm, for example, as the layerproceeds toward output port 630. In some embodiments, both the siliconridge layer 660 and the silicon rib layer 620 may be covered by asilicon dioxide cladding layer. In some embodiments, below the siliconridge layer 660 and the silicon rib layer 620, there may be a BOX layer,which may be 2 μm thick, for example.

TM₀ mode light may be input at the input port 610. Due to verticalasymmetry, TM₀ mode light can have super modes at certain waveguidesections with widths in the rib layer 620. The TM₀ mode light can betransferred into the TE₁ mode light and then output at output port 630.The structure 600 may have one or more of a high conversion efficiency,1 dB bandwidth, high fabrication tolerance, and/or compact size.

3D FDTD simulation performance of the polarization rotator 320 is shownin FIGS. 6C and 6D. As shown in FIG. 6C, TM₀ light is input at the leftof the diagram, and was successfully transferred into two branches ofthe TE₁ mode light at the right hand side of the diagram. FIG. 6D showsthe conversion efficiency of the polarization rotator 320 as 98.78%, or−0.05 dB. As shown by FIG. 6D, the wavelength response of the rotator isflat, and the 1 dB bandwidth of the polarization rotator is far largerthan 70 nm. In some embodiments, the polarization rotator 320 may besized 24 μm by 1 μm. The FIG. 6 diagrams may correspond to such a sizeddesign.

FIGS. 7A-7D show exemplary tolerance simulation diagrams of an exemplarypolarization rotator 320. FIG. 7A shows that the conversion efficiencyof the polarization rotator 320 may change less than 4.5% as the ridgewaveguide thickness of polarization rotator 320 varies from −10 nm to+10 nm. FIG. 7B shows that conversion efficiency may change less than3.5% as the ridge waveguide width of the polarization rotator 320 variesfrom −30 nm to +30 nm. FIG. 7C shows the conversion efficiency maychange less than 6% as the rib waveguide thickness of the polarizationrotator 320 varies from −10 nm to +10 nm. FIG. 7D shows the conversionefficiency changes less than 1% as the rib waveguide width of thepolarization rotator 320 varies from −30 nm to +30 nm. Thus, the data ofFIGS. 7A-7D shows that the exemplary polarization converter 320 may behighly tolerant.

FIGS. 8A-8C show diagrams relating to a mode order converter. FIG. 8Ashows an example structure 800 of an asymmetric mode converter, as anexample of the asymmetric waveguide taper mode order converter 330 shownin FIG. 3. The upper edge of the mode converter is not symmetric to thelower edge along the propagation direction. Taking the middle point ofthe input port as a starting point and drawing a horizontal line throughthe example structure 800, the upper half of the mode converter isdesigned to be wider than the lower half. The left port of the examplestructure 800 can be connected to the polarization rotator to receivethe TE₁ mode. In the asymmetric mode converter, the two lobes of the TE₁mode light input into the structure 800 may have experienced differentphase changes due to its shape and may be merged into a TE₀ mode, whichis output by the structure 800.

There is no analytic equation to describe what specific shape thisasymmetric mode converter should be. Tradeoffs also exist in the designof the structure 800, such as low optical loss versus flat spectrum andcompact size versus high fabrication tolerance. Hence, the PSO algorithmcan again be used to reach a final optimal design. The design of thestructure 800 may be divided into 20 sections along the propagationdirection and the width of each section is optimized with particle swarmoptimization (PSO) to realize high conversion efficiency, large 1 dBbandwidth, high fabrication tolerance and compact size, as shown in FIG.8A.

The example structure 800 of the asymmetric waveguide taper mode orderconverter 330, as shown in FIG. 8A, may be fabricated on a silicon layerand may be covered with silicon dioxide cladding. In one example, thesilicon layer may be at or about 220 nm thick. Below the silicon layermay be a BOX layer. In one example, the BOX layer may be at or about 2μm thick. The asymmetric layout layer is the 220 nm thick silicon riblayer. Beside the rib layer, no other silicon layer with differentthickness is used.

FIG. 8B shows exemplary 3D FDTD simulation performance of the examplestructure 800 of the asymmetric waveguide taper mode order converter330. As shown, at the left side of FIG. 8B, TE₁ mode light may be inputand transferred to TE₀ mode light, which is at the right side of FIG.8B. FIG. 8C shows exemplary polarization converter conversion efficiencyversus wavelength. As shown, the conversion efficiency is 99.34%, or−0.03 dB. The 1 dB bandwidth of the polarization converter is far largerthan 70 nm. In some embodiments, the converter 330 may be sized 21 μm×3μm. FIGS. 8 and 9 diagrams may correspond to such a sized design.

FIGS. 9A and 9B show exemplary simulated fabrication tolerance of theexample structure 800 of the asymmetric waveguide taper mode orderconverter 330. As shown by FIG. 9A, conversion efficiency changes lessthan 1 percent as the waveguide thickness varies in a range of ±10 nm.Similar results may occur as the waveguide width varies within the rangeof ±30 nm, as indicated in FIG. 9B. These simulations therefore exhibitsuitable fabrication tolerance for the structure 800.

FIGS. 10A-10E show diagrams related to the simulation of the integratedoptical apparatus 300 in accordance with some embodiments of the presentdisclosure. Performance was simulated in the wavelength range of 1270 nmto 1340 nm. FIG. 10A shows the entire structure 1000 of the integratedoptical apparatus 300 shown in FIG. 3. In some embodiments, thestructure 1000 can be sized at or about 89 μm by at or about 8 μm, suchthat it has an area of 712 μm². FIG. 10A shows the incoming light signalat the input waveguides 410 that may be an unknown mix of TE₀ and TM₀polarized light. The MMI polarization splitter 420 reflects thestructure of splitter 310 shown in FIG. 3. Structure 600 reflects thestructure of the polarization rotator 320 shown in FIG. 3. Structure 800reflects the structure of converter 330 shown in FIG. 3.

As shown by FIG. 10B, when TM₀ mode light is input as shown at the leftof the diagram, it may be converted to TE₀ mode light and exported viaan output port, as shown in the upper right of the diagram. For example,as shown by FIG. 10C, the conversion efficiency may be around 80 to 90%in the range of 1270 nm to 1340 nm (i.e., 0.5 to 0.9 dB). The 1 dBbandwidth is larger than 70 nm. The extinction ratio is over 25 dB at1310 nm and may degrade to between 9 and 10 dB at the edge of thespectrum, for example. FIG. 10D shows that when TE₀ mode light is inputas shown at the left of the diagram, it may remain as TE₀ mode light andis exported at an output port, as shown in the lower right of thediagram. For example, as shown by FIG. 10E, the transmission can bearound 71-90% in the range of 1270 nm to 1340 nm (i.e., 0.5 to 1.4 dB).The 1 dB bandwidth is larger than 70 nm. The extinction ratio is over 25dB at 1310 nm and can degrade to 10 dB at the edge of the spectrum.

It should be noted that the discussion thus far has focused onapplicability of the PSR in FIG. 3 to receive data and process the lightsignal such that it can be received by a receiver. The main applicationof this PSR would be to build a polarization insensitive receiver or apolarization MUX transmitter.

FIG. 11 is a schematic diagram of an illustrative polarizationinsensitive WDM receiver (RX) 1100. The WDM RX 1100 includes apolarization splitter 1110, which can be embodied as the opticalapparatus 300 shown in FIG. 3. As shown, the polarization splitter 1110is used to split incoming signals 1101 with two mixed orthogonalpolarization states (TE₀ and TM₀), send TE₀ to the TE de-MUX 1120,rotate the TM₀ into another TE₀ mode 1113 and send it to another TEde-MUX 1130. Then each signal is de-multiplexed by a 1×4 WDMde-multiplexer into separate constituent wavelength signals, havingwavelengths given by λ₁, λ₂, λ₃ and λ₄, where λ₁<λ₂<λ₃<λ₄. The variouswavelength signals arrive at respective bi-directional PD. The routingwaveguides 1121, 1122, 1123, and 1124 for TE₀ and 1131, 1132, 1133, and1134 for TE′₀ (converted from TM₀) are carefully arranged such that thesignals from both sides arrive at PD at the same timing. The power fromboth sides does not have to be equal. The ratio between them can be anyvalue. The bi-directional PD collects a signal from both polarizationsfor each wavelength, making the entire RX system polarizationinsensitive. In other embodiments, numbers of discrete wavelengths otherthan 4 may be used. In different embodiments, using N wavelengths, whereN is greater than one, the wavelengths λ₁ for 1≤i≤N are all differentfrom each other.

The optical apparatus 300 may also be used in a polarization-muxtransmitter. This concept operates the PSR in reverse, performing thefunction of taking two incoming TE polarized signals (TE and TE′, whichare independent channels to each other) and outputting the two signalsstreams into a single path with two orthogonal polarization states (TEand TM). In this way, laser diodes (which are commonly TE polarized), TEwaveguides, TE modulators and TE multiplexers, each built to operateonly on TE polarization, can be used with the PSR to multiplex amodulated multichannel signal stream onto a single output path. The samedesigns for the TE destined output path can be used upstream of the PSRon the TE′ destined path (then converted to TM). When used in this way,the PSR can be called a polarization rotator+combiner (PRC). To supportmultiple wavelengths simultaneously, the PRC can be broadband, offeringgood polarization extinction ratio performance and low loss over a broadwavelength range, as shown in FIG. 11

FIG. 12 shows a pol-mux transmitter system 1200. For example, light maybe generated by lasers 1210. Output light from light source 1210 may besplitted by a 3 dB power splitter. Half is modulated by modulators 1220and the other half is modulated by modulators 1230. Since modulators1220 and 1230 are driven by different RF signals, the generated signalsfrom 1220 and 1230 are independent channels to each other. Modulatedoptical signals by modulator 1220 will then be multiplexed by MUX 1240.The other optical signals generated by modulator 1230 will bemultiplexed by MUX 1250 and then converted to TM polarization by thepolarization rotator in 1260. Then, the multiplexed TE signals atwavelength λ_(A), λ_(B), λ_(C), λ_(D) from 1240 will be combined withthe TM signals at wavelength λ_(A), λ_(B), λ_(C), λ_(D) by thepolarization combiner in 1260. Finally the signals of eight independentchannels (TE_A/TE_B/TE_C/TE_D/TM A/TM_B/TM_C/TM_D) will be output intothe fiber system. As mentioned above, the PSR presented here is designedto be working for broadband. Hence, much more wavelength channels couldbe easily added and not limited to only 4 wavelengths.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Further, although the present disclosure hasbeen described herein in the context of at least one particularimplementation in at least one particular environment for at least oneparticular purpose, those of ordinary skill in the art will recognizethat its usefulness is not limited thereto and that the presentdisclosure may be beneficially implemented in any number of environmentsfor any number of purposes.

The invention claimed is:
 1. An optical apparatus comprising: a curvedmulti-mode interference (MMI) polarization splitter configured to splita light signal into a first signal having a first polarization and asecond signal having a second polarization; a polarization rotatorconfigured to rotate the second polarization of the second signal into athird polarization; and a polarization mode converter configured toconvert the third polarization of the second signal into the firstpolarization.
 2. The optical apparatus of claim 1, wherein thepolarization rotator comprises an input port and an output port, whereinthe output port is wider than the input port.
 3. The optical apparatusof claim 2, wherein the input port is at or about 420 nm wide, and theoutput port is at or about 1000 nm wide.
 4. The optical apparatus ofclaim 1, wherein the polarization rotator comprises a rib layer and aridge layer.
 5. The optical apparatus of claim 4, wherein the rib layeris at or about 90 nm thick.
 6. The optical apparatus of claim 4, whereinthe ridge layer is at or about 220 nm thick.
 7. The optical apparatus ofclaim 4, wherein the rib layer is at or about 1000 nm wide.
 8. Theoptical apparatus of claim 1, wherein the polarization mode converter isan asymmetrical waveguide taper mode converter.
 9. The optical apparatusof claim 1, wherein the first polarization is a zero ordertransverse-electric (TE) mode polarization and the second polarizationis a zero order transverse-magnetic (TM) mode polarization.
 10. Theoptical apparatus of claim 1, wherein the third polarization is a firstorder transverse-electric (TE) mode polarization.
 11. The opticalapparatus of claim 1, wherein i) the first signal having the firstpolarization and ii) the second signal having the third polarizationconverted into the first polarization are output to one or morereceivers.
 12. A method of light transmission comprising: splitting,with a curved multi-mode interference (MMI) polarization splitter, alight signal into a first signal having a first polarization and asecond signal having a second polarization; rotating the secondpolarization of the second signal into a third polarization; andconverting the third polarization of the second signal into the firstpolarization.
 13. The method of claim 12, wherein the first polarizationis a zero order transverse-electric (TE) mode polarization and thesecond polarization is a zero order transverse-magnetic (TM) modepolarization.
 14. The method of claim 12, wherein the third polarizationis a first order transverse-electric (TE) mode polarization.
 15. Themethod of claim 12, wherein i) the first signal having the firstpolarization and ii) the second signal having the third polarizationconverted into the first polarization are output to one or morereceivers.
 16. An optical apparatus comprising: a polarization modeconverter configured to convert a first polarization of a first lightsignal into a second polarization; a polarization rotator configured torotate the second polarization of the first light signal into a thirdpolarization, wherein the polarization rotator comprises a rib layer anda ridge layer; and a combiner configured to form a combined light signalby combining the first light signal having a third polarization with asecond light signal having the first polarization.
 17. The opticalapparatus of claim 16, wherein the combined light signal is input intoone or more optical transmission fibers.
 18. The optical apparatus ofclaim 16, wherein the first light signal and the second light signal aregenerated by a laser light source.
 19. An optical apparatus comprising:a splitter configured to split a light signal into a first signal havinga first polarization and a second signal having a second polarization; apolarization rotator configured to rotate the second polarization of thesecond signal into a third polarization, wherein the polarizationrotator comprises an input port and an output port, wherein the inputport is at or about 420 nm wide, and the output port is at or about 1000nm wide; and a polarization mode converter configured to convert thethird polarization of the second signal into the first polarization. 20.An optical apparatus comprising: a splitter configured to split a lightsignal into a first signal having a first polarization and a secondsignal having a second polarization; a polarization rotator configuredto rotate the second polarization of the second signal into a thirdpolarization, wherein the polarization rotator comprises a rib layer anda ridge layer; and a polarization mode converter configured to convertthe third polarization of the second signal into the first polarization.